1. Field of the Invention
The present invention relates to a process for the preparation of an optical disk to be used for a disk memory or the like. More particularly, the present invention relates to a process for the preparation of a substrate of an optical disk, in which spiral or concentric guide grooves formed on a duplicating mold such as a stamper are exactly duplicated, the adhesion of a liquid resin mixture layer to a transparent substrate is greatly improved, and if a heat resistance test is carried out after a metallic recording layer is formed on a layer of a cured product of the liquid resin mixture, no wrinkles are formed on the recording layer.
2. Description of the Related Art
Disk substrates capable of recording, reproducing, and erasing light information are generally formed by etching signal pits and guide grooves(pre-grooves) for tracking an optical head with a high precision on the surface thereof.
As a means for a mass-production of optical information-recording media having fine signal pits or guide grooves formed thereon, a method has been adopted in which a mother block having signal pits or guide grooves formed thereon is first prepared and a plastic material is injection-molded or compression-molded by using this block as a mold.
However, this injection molding method or compression molding method is defective in that a large-size molding machine must be used and the transfer precision is poor.
As a means of overcoming these defects, a duplication method in which signal pits are formed on a radiation-curable resin layer has been studied by Polygram Co. and Phillips Co., and the results of their researches are disclosed in Japanese Unexamined Patent Publication No. 51-140601 (Polygram), Japanese Unexamined Patent Publication No. 53-116105 (Phillips), Japanese Unexamined Patent Publication No. 54-130902 (Phillips), Japanese Examined Patent Publication No. 54-138406 (Phillips), and Japanese Unexamined Patent Publication No. 55-4793 (Phillips).
According to this method, a liquid radiation-curable resin layer is thinly formed on the surface of a duplicating mold such as an original disk or a stamper (a metal mold formed by duplication of the original disk by electrocasting), a colorless transparent plastic substrate is placed on this resin layer, and the assembly comprising the resin layer gripped between the duplicating mold and the substrate is irradiated with radiation to cure the resin layer. Then, the cured resin layer having signal pits duplicated thereon, and the plastic substrate integrated with the cured resin layer, are peeled from the duplicating mold to obtain an information-recording medium.
However, the adhesion between the radiation-curable resin and the plastic substrate is generally poor.
When a heat-setting resin substrate having properties desirable for a light information-recording medium, such as a broad application temperature range and an ability to resist warping by an absorption of moisture, is used as the plastic substrate, the adhesion to the radiation-curable resin layer is especially poor.
As a means for obtaining a complete adhesion, a method can be considered in which the radiation-curable resin is modified and given a soft structure. However, where a soft structure is given to the radiation-curable resin, if a metallic recording layer, for example, a photomagnetic recording layer, is formed on the radiation-curable resin layer by vacuum deposition, sputtering or the like, and a heat resistance test is carried out, fine wrinkles are easily formed on the recording layer, although the cause of this is unknown. The formed wrinkles contribute to a generation of noise during recording and reproduction, and the signal-to-noise ratio (SN or CN ratio) is drastically reduced.